[PSUBS-MAILIST] Oil Compensator

Wed Apr 5 16:02:57 EDT 2017

Jon,Cliff has been doing a lot of work on Minn kota modifications & has the handson experience. If he's not tuning in it would be worth contacting him privately.The Minn-kota seal set up is not ideal, with the gap between the two sealsmeaning one or the other seals will fail when their pressure rating is exceeded.The outer seal seems to serve a double purpose, being the first line of defenseagainst water ingress & also a sacrificial seal that will see all the abrasive muck stirred up by the prop.Noticed in this kit that the outer seal has a shield to protect it.Historically in deep diving set ups they have had dual seals, but with either ambient pressure between them,or a cascaded pressure system, and normally mechanical seals. The outer seal was regarded as sacrificial.   Maybe you could pack between the seals with grease to reduce the air gap. I am not sure whether you could simply reverse the inner seal or not as they need a shoulder that the pressurewill push them up against.Cliff is using a small relieving air regulator to compensate his motors. The regulator is fed air from your tanks& is set at an overpressure of about 4psi. The regulator is orientated upside down as the relieving air feedsout through the handle portion & the orientation will stop water entering when this valve opens.This system was suggested by Hugh, & Cliff is trialing it. It has the advantage that you could either air or oilcompensate. If your motor ran out of oil it would just be replaced with air above ambient pressure.I am not sure at what pressure the relieving valve opens.   There is this very good Psub resource from an expert in modifying Minn kotas for underwater use.Personal Submersible: White Paper: Minn Kota Motor NotesCheers Alan

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      From: Sean T. Stevenson via Personal_Submersibles <personal_submersibles at psubs.org>
 To: Personal Submersibles General Discussion <personal_submersibles at psubs.org> 
 Sent: Thursday, April 6, 2017 7:10 AM
 Subject: Re: [PSUBS-MAILIST] Oil Compensator

The best practice may not be easily achievable with a Minn-Kota without extensive modification. That said, I would orient an inner seal with lip inwards, and an outer seal with lip outwards. Pressure compensate the motor cavity with oil at a bias pressure above ambient, and separately pressure compensate the inter-seal cavity with oil at exactly ambient pressure. This way, you are either changing pressure or changing fluid across any single seal, but not both simultaneously. Minor leakage across either seal in this implementation is inconsequential. Both compensation mechanisms should have some reserve volume to account for thermal expansion of the oil.Sean

On April 5, 2017 11:49:51 AM MDT, Jon Wallace via Personal_Submersibles <personal_submersibles at psubs.org> wrote:

I haven't designed mine yet Rick which is why I'm asking the question.  
I guess the first question is whether one of the seals can be reversed 
to prevent oil from escaping around the motor shaft.  I think I recall 
Hank mentioning something about this but I don't know if it was on a 
minn-kota.  I'm also not sure if oil egress is due to heat/pressure 
build up or because the motor is designed to keep water out, not oil in.

We've got different people saying different things so I'm hoping we can 
come up with a "best practice" solution and document it in a white paper.

Jon

On 4/5/2017 1:23 PM, Rick Patton via Personal_Submersibles wrote:

 Jon

 What type of system did you use for the oil to go to when it gets hot?

 Rick

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